impregnation of aluminum casting 1

[dho]

the final product will be anodized.
so, my question is which process first, anodize or impregnation?
thanks.

 

[WKTaylor]

dho... I had same question years ago... hope this makes sense [and that I have included everything]...

o Here is the sequence I was advised to use for taking new/raw/as-cast [aluminum/magnesium alloy] castings to finished parts... and reasons 'WHY'.

NOTE.
See CAUTION at end of this statement for related concerns.

1. Visual and X-Ray inspect raw [annealed or as-cast] castings. May accomplish FPI or MPI to entire casting IF/AS directed by drawing or casting specification.

2. Accomplish any repair welding IF/AS allowed by specification and/or customer [contract, MRB, etc].

3. Heat-treat casting to final temper.

4. Machine to final dimensions/configuration. Apply any stamped/etched markings at this time [serial numbers, etc].

5. FPI, MPI, etc machined surfaces IF/AS directed by drawing; and pressure test [IF/AS required].

6. Impregnate casting [and PM] parts, Typical: MIL-STD-276 IMPREGNATION OF POROUS METAL CASTINGS AND POWERED METAL COMPONENTS, using MIL-I-17563 IMPREGNANTS FOR CAST OR POWDER METAL COMPONENTS.

7. Abrasive clean entire surface to be coated using fine glass-beads, ceramic-beads or non-metallic grit, typical: MIL-STD-1504 ABRASIVE BLASTING.

8. Penetrant inspect IF/AS required. Use of etchants is permitted on fully-machined, then sealed, parts.

9. Apply inorganic coatings to bare metal, IE: anodic [sealed] or conversion coatings.

10. Apply corrosion protective epoxy primer and epoxy or polyurethane topcoats.

o Reason 'WHY' is pretty straight forward: impregnation of [aluminum/magnesium] to seal casting porosity is the critical operation to prepare the casting to accept potentially corrosive inorganic surface finishes. Application of most conventional inorganic surface treatments... including cleaners, etchants, anodic coatings or conversion coatings, etc... exposes the casting parts to highly acidic/basic fluids which can destroy unsealed parts from the inside-out by wicking into porosity. The sealing process [if done properly] eliminates pathways for these chemicals to enter the metal pores.

o Here is some direct excerpts from MIL-A-8625 ANODIC COATINGS FOR ALUMINUM AND ALUMINUM ALLOYS, applicable to this discussion.

3.6 Class 2. When class 2 is specified in the contract or purchase order
(see 6.2), the anodic coating shall be uniformly dyed or pigmented by exposure
to a solution of a suitable type dye or stain. The color on wrought alloys
shall be uniform. Cast alloys may exhibit dye bleed-out or lack of color (or
color uniformity) associated with the inherent porosity of the casting. The
dyes and pigments used shall not be damaging to the anodic coatings.
xxxx

3.6.1.1 Casting alloys. Dyed casting alloys may shows slight lack of
color uniformity. The degree of non-uniformity that is acceptable shall be
established by the procuring activity (see 6.2).
xxxx

6.2 Acquisition requirements. ... ...
h. Degree of non-uniformity of dyed casting alloys (see 3.6.1.1).
xxxx

6.4 Electrolytic action. Severe attack by the electrolyte on castings or
welds may be occasioned either by unsound castings, Improper welding practice,
a difference In composition between the weld and the base metal or,
particularly in the case of the sulfuric acid process, the retention of the
solution in cracks, crevices, or Irregular surfaces. Severe attack by the
electrolyte may also be caused by contaminants in the electrolyte,
particularly chlorides or by improper racking of the parts.
xxxx

o CAUTION.
Chemical cleaners, HT quenchants [oil, water-polymer, etc], moisture, etchants, FPI or MPI dyes, etc can be wicked/entrapped in gross metal porosity of the raw casting, presenting serious problems for repair-welding and heat treatment. All traces of moisture and/or organic cleaners MUST be removed prior to exposure to temperatures exceeding ~200F. 'Explosive' steam formation and/or flammable/toxic/corrosive gas expulsion can occur, which will ruin castings.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
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[dho]

thanks a lot.